Journal of Alloys and Compounds 487 (2009) 263–268
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Journal of Alloys and Compounds
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pherical shape of �-Mg17Al12 precipitates in AZ91 magnesium alloy processed byqual-channel angular pressing
.N. Braszczynska-Malik ∗
zestochowa University of Technology, Institute of Materials Engineering, Al. Armii Krajowej 19, 42-200 Czestochowa, Poland
r t i c l e i n f o
rticle history:eceived 5 May 2009eceived in revised form 15 July 2009
a b s t r a c t
The microstructure investigations of the AZ91 alloy after equal-channel angular pressing (ECAP) werepresented. Solution annealed billets were processed at a 0.16 mm s−1 pressing rate and temperatures of623 and 553 K. The average grain size was reduced from 150 �m initially to a final value of 10 �m. The
ccepted 17 July 2009vailable online 25 July 2009
eywords:agnesium alloy
recipitation
microstructure analyses revealed �-Mg17Al12 precipitates with a spherical morphology located partic-ularly inside the equiaxal grains of an �-solid solution. Additionally, different shapes of � precipitates,plate-like or rod-like (typical for heat treatment processes) were not observed. The spherical shape of� precipitates was obtained due to correlate magnesium matrix alloy deformation during pressing withprecipitation process.
icrostructurequal-channel angular pressing
. Introduction
The AZ91 (Mg–Al–Zn) alloy is the most widely used magnesiumlloy exhibiting a good combination of high strength at room tem-erature, good castability and excellent corrosion resistance [1–4].he microstructure of as-cast AZ91 alloy is generally characterizedy a solid solution of aluminium in magnesium (an �-Mg phaseith a hexagonal closely-packed, hcp structure) and an � + � eutec-
ic. In Mg–Al type alloys, the � phase (called also � phase [5–9])s an intermetallic compound with a stoichiometric compositionf Mg17Al12 (at 43.95 wt.% Al) and an �-Mn-type cubic unit cell.he zinc that is present in the AZ91 alloy does not create newhases but substitutes aluminium in the �-Mg17Al12 phase, formingternary intermetallic compound Mg17Al11.5Zn0.5 or Mg17(Al,Zn)12
ype [9,10].Magnesium–aluminium alloys are susceptible to heat treatment
ue to the variable solubility of aluminium in a solid state from2.9 wt.% Al at an eutectic temperature of 710 K to about 2.9 wt.%l at 473 K. During conventional heat treatment of the AZ91 alloy,
nvolving solution annealing at about 690 K for a minimum of 24 hFig. 1a), followed by ageing at about 430 K for 16 h (T6 conditions),he discontinuous precipitates of the � phase appear [4,10]. On the
ther hand, during the ageing of a supersaturated solid solution athigher temperature, continuous precipitates of the � phase can
lso occur simultaneously [4,10]. Fig. 1b and c presents a discon-inuous and continuous precipitates of the � phase after ageing
a supersaturated AZ91 alloy at temperatures of 423 and 623 K,respectively. At intermediate ageing temperatures both discontin-uous and continuous precipitates can occur. It should also be notedthat both discontinuous and continuous precipitates in Mg–Al typealloys have plate-like morphology with an accurate orientationrelationship (OR) with a matrix phase. For both precipitates, thepredominant orientation relationship is the Burgers OR, namely:(0 0 0 1)� || (0 1 0)� and [2 1 1 0]� || [1 1 1]� [7,11–14]. Additionally,other ORs were also reported in Mg–Al-based alloys, i.e. the Craw-ley OR [7,13,14], the Porter OR [7], the Gjömmes-Östrmoe OR [13,14]or the Potter OR [14,15].
The AZ91 alloy (and other Mg–Al type alloys) is characterizedby a large grain size after heat treatment due to the long timerequired to obtain structure changes (slow volume diffusion of alu-minium in magnesium) [4]. On the other hand, grain refinementis very important in magnesium alloys because they have poorformability and limited ductility at room temperature rooted intheir hexagonal closely-packed crystal structure. In the past decade,efforts have been concentrated on thermomechanical processingfor grain size refinement using methods of severe plastic defor-mation (SPD) [16]. SPD techniques, such as equal-channel angularpressing (ECAP) [16,17], accumulative roll bonding (ARB) [18,19]or high-pressure torsion (HCP) [20,21], have been applied to thegrain refinement of magnesium alloys on bulk materials. Recentresults have shown that ECAP processing can be used to achieve an
ultrafine grain size in Mg-based alloys [22–30]. The microstructureand properties of materials pressed by ECAP are strongly depen-dent on the plastic deformation behaviour during pressing whichis governed mainly by a die geometry and process variables liketemperature, pressing speed and rotation of the billet around its
ig. 1. Microstructure of AZ91 alloy after: (a) solution annealing at 693 K for 24 h, �upersaturated solid solution; light microscopy, (b) ageing at 423 K for 16 h, discon-inuous precipitates of � phase, (c) ageing at 623 K for 8 h, continuous precipitatesf � phase; SEM.
ongitudinal axis between adjacent passes. In practice, there areour separate processing routes in ECAP: route A in which the sam-le is not rotated between passes, route BA in which the sample
s rotated through 90◦ in alternate directions between each pass,oute BC in which the sample is rotated by 90◦ in the same direc-ion (clockwise) between each pass and route C where the samples rotated by 180◦ between passes [16,17]. There are several reportsescribing the microstructure and properties of Mg–Al type alloys
rocessed by ECAP [22–30] but these investigations were not con-erned with precipitation manner. Precipitates of the � phase haveeen reported sporadically in the microstructure of the AZ91 alloyrocessed by ECAP. Chen et al. [31] observed formation of � phaserecipitates at grain boundaries of the AZ91 alloy during two-step
and Compounds 487 (2009) 263–268
ECAP deformation carried out at 498 (four passes) and 453 K (twopasses) at a pressing speed of 25.2 mm/min and via route BC (beforeECAP samples were hot-rolled). Máthis et al. [32] observed rod-likeprecipitates in AZ91 alloy pressed four times at 543 K using routeC with a pressing rate of 5 mm/min. They also concluded that longrod-like precipitates were broken into smaller parts during ECAPcarried out in up to eight passes. Miyahara et al. [26] presentedsmall � phase particles lying preferentially on the grain boundaryin the AZ61 alloy after pressing in four passes at 473 and 523 K viaroute BC.
In the present work, ECAP processing was applied to obtaina new shape of � phase precipitates in AZ91 alloy thanks tocorrelate plastic deformation behaviour during pressing with a pre-cipitation process. Investigations of the individual microstructuredevelopment were conducted using light, scanning and transmis-sion electron microscopy.
2. Experimental procedures
The commercial as-cast AZ91 magnesium alloy with a nominal chemical com-position of 9 wt.% Al, 1 wt.% Zn, 0.5 wt.% Mn was used in this study. The investigatedAZ91 alloy was cut into rods of 50 mm in length and 11.8 mm in diameter. Beforethe ECAP process, the samples were heat treated in order to obtain a homogeneousmicrostructure. Solution annealing was carried out at 693 K for 26 h in a protectiveargon atmosphere followed by water quenching (at approximately 287 K) for all thesamples. The microstructure obtained after this heat treatment was characterizedby large grains of supersaturated solid solution with an average grain size of about150 �m (Fig. 1a).
The ECAP die used in this investigation was designed to obtain a maximum shearstrain of about 1.15 during each pass. It contained an inner contact angle ˚ equalto 90◦ and corner angle � of 0◦ . The billets were processed at a pressing rate of0.16 mm s−1 using a plunger attached to a hydraulic press on an Instron machine. Allthe pressings were conducted using route BC. This procedure was adopted because itis evident from many experiments [16,17,23–30] that the BC route leads most effec-tively to an array of equiaxed, fine grains separated by high-angle grain boundaries.For each separate pressing, the samples were coated with molybdenum disulphide(MoS2) as a lubricant. The temperature of the process was controlled using a ther-mocouple in a die. The process was conducted at 623 and 553 K, i.e. below the solvuscurve (in the range of ageing temperatures).
The samples were subjected up to four pressings and then sectioned formicrostructure examination. All microstructure analyses were carried out from bothtransverse and longitudinal sections of the as-pressed billets using light and scan-ning electron microscopy (SEM) techniques. A standard metallographic techniquewas applied for sample preparation including wet prepolishing and polishing withdifferent diamond pastes without contact with water. To reveal the microstruc-ture, samples were etched in a 1% solution of HNO3 in C2H5OH for about 60 s. AnXL30ESEM FEG (Philips, Eindhoven, The Netherlands) scanning electron microscopewas used. The microstructure of the deformed samples was also studied using trans-mission electron microscopy (TEM) on a Philips CM20 instrument equipped withan energy dispersive X-ray (EDX) spectrometer operating at 200 kV (Philips, Eind-hoven, The Netherlands). For TEM analyses, the thin foils for transmission electronmicroscopy study were polished electrolytically. The slices of material from boththe transverse and longitudinal sections of the as-pressed billets were mechani-cally thinned down, followed by punching of 3-mm diameter discs. Finally, the discswere thinned to perforation using a Fischione double-jet electropolisher (Fischione,Export, PA) with a solution of 33% nitric acid and 67% methyl alcohol at a temperatureof 245 K and a voltage of 20 V. The brightfield technique was used for microstructureobservations and selected area electron diffraction (SAED) patterns were employedto define structural constituents.
3. Results
Fig. 2 shows a representative microstructure of the AZ91 alloyobserved after four passes of the ECAP process conducted at 623 K.The initial average grain size for the annealed condition was about150 �m as shown in Fig. 1a. The grain structure shown in Fig. 2was recorded after four passes via route BC using the die withcontact angle ˚ equal to 90◦ (which allowed attainment of a maxi-
mum shear strain of about 1.15 during each pass). The average grainsize was reduced to a final value of about 10 �m. This result con-firms that ECAP is a powerful technique to obtain a fast decrease ofgrain size in the AZ91 magnesium alloy. Additionally, it can also beseen that the microstructure was characterized by the presence of
K.N. Braszczynska-Malik / Journal of Alloys and Compounds 487 (2009) 263–268 265
Fs
eo
EilFtT6tbot(tmtm
PpdcaZcfczfi
ig. 2. Microstructure of AZ91 alloy after four ECAP passes at 623 K (a) transverseection (b) longitudinal section; light microscopy.
quiaxal grains in both the transverse and the longitudinal sectionsf the pressed billets.
The microstructure observation of the AZ91 alloy processed byCAP at 623 and 553 K also revealed the presence of fine, spher-
cal precipitates located especially inside the grains. Fig. 3 showsight microscopy images of the investigated alloy pressed at 553 K.ine precipitates with a new spherical shape were observed in bothhe transverse and the longitudinal sections of the pressed billets.he same result was obtained after the ECAP process conducted at23 K. Fig. 4 presents SEM images with visible spherical precipi-ates of the AZ91 alloy after ECAP processing at 623 K. It should alsoe noted, that different types of precipitates, i.e. with plate-liker rod-like morphology were not observed. In the microstruc-ure of the AZ91 alloy after the conducted ECAP process, typicalfor heat treatment) continuous or discontinuous precipitates ofhe � phase (Fig. 1b and c) with characteristic morphology and
arked anisotropy of growth were not present. Identification ofhe observed precipitates with a spherical shape was carried out by
eans of the transmission electron microscopy technique.Figs. 5–7 show typical TEM images of the analysed material.
recipitates with spherical morphology are clearly visible in botherpendicular sections of billets pressed at 553 and 623 K. Energyispersive X-ray spectrometry analyses obtained from the pre-ipitate marked as X in Fig. 6a revealed a high concentration ofluminium, i.e. 37 wt.% Al and raised content of zinc, i.e. 1.9 wt.%n. Although the Mg:Al ratio recorded exceeds the expected stoi-hiometry of Mg17(Al,Zn)12 phase, probably because of influences
rom the matrix, presented result indicates that the observed pre-ipitates are the � phase and it also provides direct evidence ofinc presence in the � phase. Spherical precipitates were identi-ed as the � phase from the analysis of the selected area diffraction
Fig. 3. Microstructure of AZ91 alloy after two ECAP passes at 553 K (a and b) trans-verse section (c) longitudinal section; light microscopy.
pattern. The associated SAED patterns obtained from the sphericalprecipitates, marked as Y and Z in Fig. 7a and b, respectively, confirmthe identification of the � phase. Additionally, the conducted inves-tigations did not disclose any difference in structure parameters(like unit cell parameters) between the newly obtained sphericalprecipitates and typical plate-like precipitates of the � phase anal-ysed earlier [33]. It should also be noted that the maximum size ofthe obtained spherical � precipitates was about 1 �m.
4. Discussion
The presented results indicated that the microstructure of the
AZ91 alloy processed by ECAP at 553 and 623 K via the BC press-ing route at a rate of 0.16 mm s−1 consisted of � matrix phaseequiaxal grains of about 10 �m in size (Figs. 2 and 3) and sphericalprecipitates located particularly inside the grains (Figs. 3 and 6).
266 K.N. Braszczynska-Malik / Journal of Alloys and Compounds 487 (2009) 263–268
Fs
Tmtdodote
F
ig. 4. Microstructure of AZ91 alloy after four ECAP passes at 623 K (a) longitudinalection (b) transverse section; SEM.
he observed precipitates were identified as the � phase. Theicrostructure observations carried out on both the transverse and
he longitudinal sections of the pressed samples provide direct evi-ence of the spherical shape of precipitates with a maximum sizef about 1 �m. It should also be noted that in deformed samples,
ifferent shapes of precipitates (plate-like or rod-like) were notbserved. It is well known that equilibrium morphology is a shapehat minimizes the total energy which is composed of two parts:lastic strain energy and interfacial energy.
ig. 5. TEM image of AZ91 alloy after two ECAP passes at 623 K; longitudinal section.
Fig. 6. TEM image of AZ91 alloy after two ECAP passes at 623 K; longitudinal section(a) and result of EDX analysis obtained from precipitate marked as X (b).
In the AZ91 alloy, discontinuous or continuous precipitates ofthe � phase growing during the ageing of a supersaturated solidsolution have characteristic plate-like morphology with markedanisotropy of growth, presented in Fig. 1a and b, respectively. Pre-cipitates with the Burgers OR are parallel to the basal plane ofthe matrix, i.e. (0 0 01)� whereas precipitates with optional ORslie on the prism plane of the magnesium, i.e. (11 00)� and theyare perpendicular to the basal plane of the matrix. The analyses ofORs [5,10–15,33] suggest the presence of coherent boundaries withslight lattice deformation between the magnesium matrix and �precipitates that explain both plate-like morphology of precipitatesand its visible anisotropy of growth.
On the other hand, the analysed planes (basal and prism planes)in hcp magnesium are simultaneously the main slip planes. Formagnesium with a c/a ratio equal to 1.624 the main slip systemis {0 0 0 1}
⟨1 1 2 0
⟩[34,35]. In case of unfavorable orientation of
the main slip system to external stress or in higher temperatures,different slip systems, e.g. {1 1 0 0}
⟨1 1 2 0
⟩and {1 1 0 1}
⟨1 1 2 0
⟩
can also operate. In works concerning hot working of magnesiumalloys [36,37] however, an 〈a〉 cross-slip and energetically favor-able junction between glissile 〈a〉 and sensile c dislocations on a{1 1 0 0} prism plane seem to be predominant. The dissolution ofan a dislocation on the basal plane to Shockley partial dislocationsconnected with a single stacking fault is also possible. However,plastic deformation during SPD processes especially of magne-sium alloys at high temperatures appears to be more compositeand complicated. For example, in the present case, the supersatu-
rated solid solution exhibited higher than equilibrium solute atomsconcentration, which could form atmospheres generating disloca-tion locking. It should also be noted that the deformation of hcpmagnesium alloys caused the formation of twins, especially the{1 1 0 2}matrix‖{0 1 1 2}twin type [38,39].
K.N. Braszczynska-Malik / Journal of Alloys a
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ig. 7. TEM images of AZ91 alloy after two (a) and four (b) ECAP passes at 623 K; lon-itudinal sections (SAED patterns obtained from the spherical precipitates markeds Y and Z).
In most published works [23–25,30,39] the microstructure ofagnesium alloys processed by the ECAP technique was charac-
erized by strong deformation and high density of dislocations andwins. Introducing a high dislocation density and strong disorderingf the matrix lattice during severe plastic deformation can precludeormation of a coherent boundary between the matrix and grow-ng � precipitates. If precipitate growth proceeds during continuouslastic deformation, then the plate-like shape of precipitates isnfavorable. In this case the spherical shape of precipitates cane the most energetically favorable. On the other hand, the highensity of crystal defects introduced inside � phase grains duringlastic deformation act as privileged sites for the heterogeneousucleation of precipitates. Additionally, high dislocation density
and other defects like vacancies) caused faster diffusion of solutetoms in the magnesium matrix which were effective in reducinghe time necessary for the nucleation and growth of precipitates (inomparison to precipitation during heat treatment). For this reason
[[[[
nd Compounds 487 (2009) 263–268 267
also, spherical precipitates were observed especially inside matrixgrains which is seen in Fig. 3.
In the present case, a new spherical shape of � precipitateswas obtained in the AZ91 alloy processed by ECAP at a maximumshear strain of about 1.15 during each pass and at a pressing rateof 0.16 mm/s. The applied ECAP parameters allowed the attainmentof severe plastic deformation introducing strong disordering of themagnesium lattice. On the other hand, the temperature and timewere suitable for the growth of precipitates during deformation.Thus, the selected process parameters allowed the attainment ofnew spherical precipitates of the � phase. The obtained results aredifferent of those described earlier [31,32,26], probably due to dif-ferent process parameters, like for example pressing speed. On theother hand, very important factor could be a time of samples heat-ing between passes. For example, prolonged holding of billets in adie between consecutive passes may causes static recrystallizationprocesses of matrix and different conditions inside grains duringnucleation and growth of precipitates, which may result in differentform of precipitates.
5. Conclusions
1. Equal-channel angular pressing is a powerful technique for theprocessing of magnesium alloys with a fine grain structure. Thehomogeneous microstructure of the AZ91 alloy with an average10 �m equiaxial grains was obtained after four passes via routeBC.
2. The spherical shape of � precipitates was obtained due to corre-late precipitation and magnesium matrix deformation processes.The high density of crystal defects (dislocations especially) intro-duced during severe plastic deformation precluded growth ofplate-like precipitates with a coherent boundary with the matrixand enforced a new energetically favorable spherical shape.
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